Known converter circuits are used in various ways. For example, direct converters have advantages for operating drives since in some implementations they can drive higher currents than indirect converters with the same effort.
Modular multi-level converters (abbreviations=MMC, MMLC or M2LC) are used, for example, as direct converters. Such converters have converter branches which connect each input-side phase line to an output-side phase line. Such converters may also be used as partial converters which are connected to one another in series.
Each of the converter branches has a series circuit including an inductor and one or more switching cells in the form of two-pole networks. The switching cells may be constructed with a half-bridge or full-bridge circuit and may have an energy store, for example a switching cell capacitor. By means of suitable connection, the converter branch can be connected in such a manner that the energy store is connected into the converter branch or is disconnected from the latter. The direct converter can be driven for each switching cell individually such that no voltage is applied to the terminals of the respective switching cell and this allows only a passive current flow through freewheeling diodes or the voltage of the energy store is dropped across the terminals of the respective switching cell, that is to say the voltage of the switching cell capacitor in the case of a half-bridge connection or the non-inverted or inverted voltage of the switching cell capacitor in the case of a full-bridge connection. Direct converters of this type are known, for example, from the document WO 03/090331 or US 2011/0075 465 A1.
Faults may occur in direct converters. If a fault occurs, an immediate reaction can be specified in order to avoid destruction of components.
The document US 2008/0232 145 A1, for example, discloses a multi-level converter in which the converter branches have a plurality of switching cells. If a fault occurs in one of the switching cells, which can be identified, for example, by comparing a measured output voltage with an expected voltage, the relevant switching cell is bridged.
The document Maharjan, L. et al., “Fault-Tolerant Operation of a Battery-Energy-Storage System Based on a Multilevel Cascade PWM Converter with Star Configuration”, IEEE Transactions on power electronics, Volume 25, No. 9, Sep. 2010, pages 2386-2396, proposes, after a fault has been identified in a switching cell of the converter, short-circuiting the switching cell, with the result that the converter can continue to be operated with the other switching cells. Faulty switching cells can be determined, for example, using changes in the output voltage profile and/or the capacitor voltage profile.
The full-bridge circuits or half-bridge circuits in the switching cells can have IGBTs as power semiconductor components for switching electrical currents. In the case of IGBT power semiconductor switches, the internal resistance suddenly increases as of a certain current intensity carried, which can be referred to as IGBT desaturation, for example. If desaturation occurs in an IGBT, the latter should be immediately switched off since the power converted in the IGBT may quickly exceed the maximum permissible power on account of the voltage increase resulting from the increase in resistance. For example, IGBTs are nowadays able to withstand the occurrence of desaturation and disconnection of the overcurrent for a predetermined period of time, for example up to 10 μs, if the intermediate circuit does not have an excessively large stray inductance. So that rapid disconnection can be carried out after the occurrence of IGBT desaturation has been determined, a switching cell control unit which is specified for this purpose and is intended to monitor the switching cell and to disconnect the IGBTs is directly provided in the individual switching cells. Only after desaturation has been identified by the switching cell control unit and the IGBT has been disconnected is a central control unit informed of the disconnection of the relevant switching cell.
So that the switching cell can quickly react to an identified fault, the individual switching cells may be provided with bridging elements, with the result that faulty switching cells can be reliably short-circuited. Thyristor circuits, electromechanical switches, pyrotechnic switches, or broken-down semiconductors are known as bridging elements. However, many proposed bridging elements may only be short-circuited once and can no longer be readily opened.
Overcurrents in IGBTs, which result in desaturation, can now be caused not only by faults in the switching cells themselves but also by other faults which occur outside the power converter or outside the switching cells.
Therefore, as long as it is reliably identified that the fault is present in the switching cell in which the bridging element is closed, the bridging element can remain closed. However, since faults which are outside the relevant switching cell are also incorrectly associated with a switching cell with the structure described above, the situation may arise in which functional switching cells are bridged and, as a result, would be rendered permanently non-functional since the short circuit across the switching cell cannot be canceled again using a simple measure. However, it has hitherto been assumed that each switching cell in which a fault is identified is defective or faults in the switching cells are identified outside the switching cells and are associated with the switching cells by complicated analysis methods.
WO 2011/116816 A1 likewise specifies a multi-level converter having switching cells, wherein a triggering element for short-circuiting switching cell connections (WO 2011/116816 A1, FIG. 5, reference symbol 18) is provided between two switching cells (WO 2011/116816 A1, FIG. 5, reference symbols 6a, 6b), wherein the triggering signal is generated by a central control unit (main controller) directly or via a cell control unit (WO 2011/116816 A1, FIG. 5, reference symbol 34) with a time delay with respect to the occurring fault and is sent to the triggering element.
“Prospects of Multilevel VSC Technologies for Power Transmission”, B. Gemmell et al, Transmission and Distribution Conference and Exposition, 2008, T&D IEEE/PES, Piscataway, N.J., USA, Apr. 20, 2008, likewise discloses a multi-level converter of the generic type having switching cells.